Professor Hinck Bio 110, problem set 3 1) Order the molecules on the following list according to their ability to diffues through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your answer. 1. Ca++ 2. CO 2 3. Ethanol 4. Glucose 5. RNA 6. H 2 O 1ANS) The order is CO2 (small and nonpolar) > ethanol (small and slightly polar) > H2O (small and polar) > glucose (large and polar) > Ca2+ (small and charged) > RNA (very large and highly charged). This list nicely illustrates the two basic properties that govern the capacity of molecules to diffuse through a lipid bilayer: size (small > large) and polarity (nonpolar > polar > charged). 2) K+ channels typically transport K+ ion 10,000 times better than Na+ ions, yet these two ions carry the same charge and are about the same size (0.133 nm for K+ and 0.095 nm for Na+). Briefly describe how a K+ channel manages to discriminate against Na+ ions. 2ANS) The key feature of a K+ channel is its selectivity filter. For the bacterial K+ channel, whose three-dimensional structure is known, the selectivity filter is a short, rigid, narrow pore, which is lined by carbonyl oxygens from the polypeptide backbone. The carbonyl groups are spaced exactly to accommodate a K+ ion, allowing it to shed its water molecules and interact with the oxygen atoms of the carbonyls. By contrast, the oxygen atoms are spaced slightly too far apart to accommodate a Na+ ion, providing too little binding energy to make up for the loss of water molecules that must be shed to pass through the filter.

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